Breitbart, a conservative Internet publisher and author, collapsed shortly after midnight while walking near his Los Angeles home, the Associated Press reported. He was rushed to Ronald Reagan UCLA Medical Center where he was pronounced dead.

According to a statement posted on his website, Breitbart died "unexpectedly from natural causes." But the hard-hitting journalist had a history of heart problems, the AP reported. No official cause of death has been given.

"People who have had heart disease at are at a higher risk of having further heart disease," said Dr. Lawrence Phillips, a cardiologist and assistant professor of medicine at NYU Langone Medical Center in New York. Phillips does not know the details of Breitbart's previous heart condition or any treatment he received, but said, "It's tremendously important that after being diagnosed with heart disease, you have close follow-up with a cardiologist."

More than 27 million Americans have heart disease, which is the leading killer of both men and women. The American Heart Association recommends that adults over 40 know their risk factors, which include smoking, high blood pressure, high blood cholesterol, diabetes, being overweight or obese, physical inactivity and a family history of heart disease.

"Any time we see someone young die of heart disease, we always look to see what risk factors they had," said Phillips, adding that heart disease can strike anyone -- even a person without risk factors -- at any time. "We can't stop it from happening, but we can decrease the odds."

Increasing age is another risk factor. But heart disease killed 150,000 Americans under the age of 65 in 2007, according to the U.S. Centers for Disease Control and Prevention. Symptoms include chest pain or fluttering, irregular heartbeat, shortness of breath and fainting.

The nature of Breitbart's previous heart condition, which was reportedly diagnosed last year, and whether it contributed to his death are unknown. The Los Angeles County coroner's office said an autopsy would "more than likely" be conducted, the AP reported.

Breitbart is survived by his wife, Susie, and four young children.

"With a terrible feeling of pain and loss we announce the passing of Andrew Breitbart," read a statement posted on Breitbart's website today. "We have lost a husband, a father, a son, a brother, a dear friend, a patriot and a happy warrior."